1. Field of the Invention
The invention relates to a method for separating slices from a hard brittle workpiece, and to a wire saw which is suitable for this method. The method is suitable in particular for separating semiconductor wafers from a crystal, for example from a silicon single crystal.
2. The Prior Art
Wafers of this type must have sides which are particularly planar and parallel to one another if they are to be useable as base material for the fabrication of electronic components. Therefore, enormous effort is dedicated to satisfying the requirements. There is also considerable interest in getting as close to the objective as possible even during separation of the slices, since subsequent working steps required to improve the geometry, such as lapping or polishing, can then be made easier or even eliminated altogether.
The surfaces of semiconductor wafers which have been produced with the aid of a standard wire saw, on closer examination, have a surface structure with score marks and undulations. This unevenness has to be eliminated in subsequent machining steps, which requires a relatively large amount of material to be removed and therefore a considerable amount of time is required.
European Patent No. EP-885679 A1 has described a complex method in which the accuracy of the cutting profile is increased by pivoting the wire web out of the horizontal plane against the running direction of the wires.
It is therefore an object of the present invention to provide a method that is simple to carry out and which forms slices which require less material to be removed in subsequent machining steps.
The invention relates to a method for separating slices from a hard brittle workpiece, in which the workpiece is subjected to an advancing movement and is pressed onto wires of a wire web of a wire saw, the wires being under a defined wire tension and making contact with the workpiece over a cutting depth along a cutting contour. The wires are lifted off the cutting contour a number of times, i.e., repeatedly, while the wire tension is kept substantially unchanged. This is accomplished with the aid of rolls of a roll system, as a result of at least one roll being moved onto the wire web and back off. The roll system comprises at least two moveable rolls which flank the workpiece.
The cutting contour is formed when the wires engage in the workpiece and is of the same length as the engagement length (contact length) of a wire. It is in the shape of an arc, since the wires yield slightly under the pressure of the workpiece. The cutting depth is the maximum depth of the cutting gap which is formed when the wires penetrate into the workpiece. Unlike a conventional wire-sawing method, in which the wires remain in contact with the cutting contour over the maximum engagement length, the wires are lifted off the cutting contour a number of times, i.e., repeatedly, preferably at periodic intervals, through an angle xcex1, with the aid of the roll system. In the process, they move through the cutting gap and, by interacting with a sawing slurry or with abrasive grain which is bonded to the wires, reduce undulations and score marks at the side faces of the cutting gap.